There is known a hydraulic shovel as described above, the hydraulic shovel including a plurality of hydraulic pumps for driving respective hydraulic actuators. For example, Patent Literature 1 discloses a hydraulic shovel equipped with a hydraulic circuit as shown in FIG. 9.
Specifically, the circuit shown in FIG. 9 includes: a first pump 101, a second pump 102, and a third pump 103, each pump being a hydraulic pump driven by an engine 100; a boom cylinder 111, an arm cylinder 112, and a bucket cylinder 113 which are respective hydraulic actuators for the boom, arm, and bucket; a slewing motor 114 for slewing an upper slewing body on which the boom is installed; a boom remote control valve 121, an arm remote control valve 122, and a bucket remote control valve 123 for operating the boom, arm, and bucket, respectively; a first boom control valve B1 and a second boom control valve B2 for controlling the operation of the boom cylinder 111 according to the operation applied to the boom remote control valve 121; a first arm control valve A1 and a second arm control valve A2 for controlling the operation of the arm cylinder 112 according to the operation applied to the arm remote control valve 122; a bucket control valve BU for controlling the operation of the bucket cylinder 113 according to the operation applied to the bucket remote control valve 123; and a slewing control valve SL for controlling the operation of the slewing motor 114.
To respective discharge ports of the first to third pumps 101 to 103, connected are first, second, and third center bypass lines 141, 142, 143 running from the respective discharge ports of the first to third pumps 101 to 103 to a tank. To the first center bypass line 141 are connected the first arm control valve A1 and the second boom control valve B2 in the order of description from the upstream side along the line, so as to be arranged in a tandem; to the second center bypass line 142, connected are the bucket control valve BU, the first boom control valve B1 and the second arm control valve A2 in the order of description from the upstream side along the line, so as to be arranged in a tandem; and to the third center bypass line 143 is connected the slewing control valve SL.
Each control valve is formed of a three-position hydraulic pilot controlled selector valve having a neutral position and respective operation positions at both sides of the neutral position, configured to be shifted from the neutral portion to either of the operation positions by the operation applied to the remote control valve corresponding to this control valve. In the neutral position, each control valve forms an oil path for opening the center bypass line to which the control valve is connected; in either of the operation positions, each control valve forms an oil path for leading a part of the hydraulic oil flowing in the center bypass line to a hydraulic actuator corresponding to the control valve (for example, the boom cylinder 111).
However, the circuit shown in FIG. 9, in which the plurality of control valves are arranged in tandems along the respective first and second center bypass lines 141, 142, fails to permit the hydraulic oil to be supplied at a sufficient flow rate to the hydraulic actuator corresponding to the downstream control valve when the upstream control valve is operated with a large stroke. This causes inconvenience that the motion of the hydraulic is slowed. For example, in the second center bypass line 142, when there is applied a full operation or an operation close thereto to the first boom control valve B1 connected to the second center bypass line 142, the second arm control valve A2 positioned downstream thereof cannot be supplied with the hydraulic oil at a sufficient flow rate. This causes inconvenience is that the motion of the arm cylinder 112 connected to the second arm control valve A2 is slowed.
As means for avoiding such inconveniences, there can be conceived, for example, providing a parallel line branched off from the second center bypass line 142 upstream of the first boom control valve B1 to reach the second arm control valve A2 while so as to bypass the first boom control valve. However, this may causes the flow rate of the hydraulic oil to be biased to the second arm control valve A2 and the arm cylinder 112 upon such an operation that the drive load on the arm cylinder 112 becomes much smaller than the drive load on the boom cylinder 111 (for example, the operation of retracting the bucket above or on the ground by a combination of a boom raising operation and an arm retracting operation), which conversely hinders the boom cylinder 111 from normal operation. Avoiding this trouble requires a throttle performing a large flow-rate-restriction in the parallel line, the addition thereof involves a great increase in the pressure loss on the meter-in side.
Although the circuit shown in FIG. 9 includes the third pump 103 in addition to the first pump 101 and the second pump 102, the third pump 103 is used exclusively for slewing drive, not contributing to adequate actuations of the boom, arm, and bucket.